Dopamine and Dihydroergocryptine Binding to the Anterior Pituitary and Other Brain Areas of the Rat and Sheep*

Subcellular preparations of the anterior pituitary of rats and sheep bind 3Hdopamine (3HDA) and the dopaminergic agonist 3Hdihydroergocryptine (3HDHE) in a manner compatible with interactions at a DA receptor. The binding is high affinity (3HDA, dissociation constant = 65 nM; 3HDHE, dissociation constant = 5 nM), low capacity (3HDA, 336 fmol/mg protein; 3HDHE, 250 fmol/mg protein) and seems to be associated with a single order of binding sites. In addition, the binding is stereoselective, as the physiologically active (+)isomer of butaclamol reduces binding of 3HDA and 3HDHE by 50% at concentrations 1000 times less than the inactive (-)butaclamol. Interactions of dopaminergic and adrenergic agonists and antagonists with this binding site mimic their pharmacological potencies at a DA receptor. A similar binding site was present in the corpus striatum and the posterior pituitary, regions which contain DA axon terminals, as well as in the region of the median eminence. However, insufficient tissue was available from the posterior pituitary and the median eminence to adequately characterize the binding. The cerebellar cortex, which does not receive dopaminergic innervation, contained no detectable binding sites for either 3HDA or 3HDHE. These data strongly support the existence of a DA receptor in the anterior pituitary. In addition, the data confirms DA and DHE binding in the corpus striatum and extends the regions of such binding to include the posterior pituitary and the stalk-median eminence. Future studies must be performed to determine the cell types in the anterior pituitary upon which this binding occurs.